Vehicle operators prefer a close correspondence between steering wheel angle and vehicle steering. However, depending upon the characteristics of the vehicle, the vehicle may tend to have an increased understeer, so the operator must turn the steering wheel more to get the vehicle to properly turn. The vehicle may also oversteer, requiring the operator to turn the steering wheel less for the vehicle to properly turn. When steering a vehicle, the speed of the driven wheels of the vehicle can affect vehicle steering performance to deviate from its pre-set understeer characteristics.
When a vehicle turns, the wheels closest to the center of the turning radius are referred to as the inner wheels and the wheels farthest from the center of the turning radius are referred to as the outer wheels. Referring to FIG. 1, the vehicle 12 has four wheels 14, 18, 22, and 20, a wheel base L, and each wheel has a distance d from the center line 17 of the vehicle. The vehicle is shown with front wheels 18 and 20 turned so the vehicle makes a right turn such that the center of gravity 16 of the vehicle 12 turns at a radius R around the center of the turn 24. In this example, wheels 20 and 22 are inside wheels and wheels 18 and 14 are the outside wheels. When the vehicle turns left, wheels 18 and 14 are the inside wheels and wheels 20 and 22 are the outside wheels.
An operator negotiates a turn which has a turning radius of R.sub.d by rotating the vehicle steering wheel. However, if vehicle understeer characteristics (note: "understeer characteristics" may infer both understeer and oversteer depending on the sign) deviate from the vehicle's nominal setting, the actual turning radius of the vehicle, R, varies from the desired turning radius, R.sub.d. Vehicle understeer occurs when the steering wheel is turned a specific amount, indicating a desired steering radius R.sub.d, but the actual steering radius, R, is greater than R.sub.d. Vehicle understeer is characterized by an understeer coefficient K.sub.u, which is a function of the static normal load on the front and rear tires and the cornering stiffness of the tires. An understeer coefficient K.sub.u &gt;0 occurs during vehicle understeer, and is considered stable, however, it is not desirable to have a very large understeer coefficient.
Vehicle oversteer occurs when the steering wheel is turned to a specific amount, indicating a desired steering radius R.sub.d, but the actual steering radius, R, decreases as vehicle speed increases. This is considered an undesirable trait, and occurs when the understeer coefficient K.sub.u is less than zero.
Because the cornering stiffness of a given tire varies with a number of operational parameters including inflation pressure, normal load, braking effort, and lateral force, the understeer coefficient K.sub.u varies with the operating conditions of the vehicle. Therefore is is important to maintain the effective understeer coefficient within a certain desired level, 0 .ltoreq.K.sub.u .ltoreq.k.sub.d, to provide stable and consistent vehicle behavior, allowing the driver to direct the vehicle without excessive steering.